When nitric acid plants with catalytic combustors shutdown, they vent untreated NO.sub.2 containing process gas. This vented gas creates a red, opaque plume because of its nitrogen dioxide content, e.g. vented gas containing a 200 ppm NO.sub.2 concentration from a two (2) foot diameter stack creates a plume of 10% opacity. Although currently allowed by some state regulating agencies, increasingly stringent emission plume requirements are being imposed on the industry; therefore, there is a greater need to minimize or eliminate these emission plumes.
One solution to the nitric acid plant opacity problem has been suggested in U.S. Pat. No. 4,330,520. In this patent, Roller teaches a process and apparatus to prevent NO.sub.x emissions after an emergency shutdown in a process for manufacturing nitric acid. In the process, the intake nitrous gas to the compressor is interrupted, the residual intake nitrous gases are conveyed to the delivery side, the delivery side gas volume is shut off, the gas on the delivery side is expanded to the intake side and then exhausted into a vacuum system. Subsequently, the acid charged with NO.sub.x in the absorption stages is drawn off, degassed acid is circulated through the absorption stages until equilibrium is established, the pressure of the shut off section is released and cooled acid is admitted to the absorption stages.